Knife sharpener



FIG. 1 is a top perspective view of a knife sharpener, showing my new design;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a front elevational view thereof;

FIG. 5 is a right side, elevational view;

FIG. 6 is a bottom plan view thereof;

FIG. 7 is a left side, elevational view thereof; and

FIG. 8 is a rear elevational view thereof. 

The ornamental design for a knife sharpener, as shown and described. 